In furnaces and other systems including burners for producing a flame, it is frequently desirable or necessary to monitor the burner to ascertain that a flame is, in fact, present during times when the burner is operating. Accordingly, devices have been developed for monitoring a flame and providing an output signal representative of whether or not a flame is present in the burner. Such devices find particular application in furnace systems where it is necessary to continuously monitor a flame to ensure safe operation.
For example, it sometimes happens that upon starting up a furnace, the burner does not ignite. Another occurrence which is not uncommon is a flame-out where a burner flame is extinguished during the operation of the burner. Such situations may be extremely dangerous if not promptly detected. Typically, burner control systems monitor the presence of a flame, and upon a loss of flame, the burner control system immediately shuts off the fuel supply to the burner. If such precautions are not taken, a dangerous concentration of unburned fuel and/or vapors may accumulate in the furnace and result in a fire or explosion.
Various devices and circuits have been known in the prior art for monitoring the presence of a flame. Typically, such devices include a sensor, such as an ultraviolet or infrared radiation sensor, which provides an output signal in response to radiation from a flame. The output signal from such a sensor is applied to a flame analyzer circuit which processes the signal and provides an output signal representative of whether a flame is present.
Typically, the flame sensor output signal is composed of a series of pulses. These pulses must be filtered to smooth them out and to provide a continuous signal representative of the flame quality. For safe operation, such filters must have a response time sufficiently rapid that the circuit output signal indicates a no-flame condition within a predetermined short period of time after a loss of flame.
Prior art circuits for providing the above-described filtering have employed RC or equivalent circuits to which are applied the flame sensor output signals. By choosing the proper parameters and time constants for the RC circuit, individual pulses from a flame may be smoothed out while still providing a response time rapid enough to prevent the build-up of an unsafe condition after a loss of flame.
Due to the highly critical nature of the flame detector circuitry, it is very important that such circuitry be extremely reliable. In order to verify proper operation of the entire flame evaluation circuit, a flame sensor shutter is frequently employed to periodically shield the flame sensor from the flame being monitored. Additional circuitry is provided to ascertain that pulses are not produced by the flame sensor circuitry during the interval when the shutter is closed. Such circuits are shown in U.S. Pat. Nos. 2,798,213 and 2,798,214.
While prior art circuits for evaluating the quality of a flame have generally proved to be reliable in terms of avoiding malfunctions, under certain conditions, such circuits have difficulty in distinguishing between a flame of acceptable quality and one of unacceptable quality. In view of the extreme danger of an indication that a flame is present when no flame exists, such flame detection circuits are generally designed to err on the safe side. Under marginal flame conditions or when the flame sensor does not have a good line-of-sight view of the flame, this results in annoying shutdowns of the furnace system due to an erroneous decision that no flame is present.
A similar situation can exist with a multiple burner system. In such a system, it is important to monitor the flame from each burner and to shut down a burner if its flame goes out. Generally, individual flame sensors are used to monitor each burner and are adjusted to be exposed to direct radiation from that burner only, to as great an extent as possible. However, background radiation from other burners and signals produced by flames from other burners flowing into the line-of-sight of a flame sensor may result in output pulses from the flame sensor even though its burner has been extinguished. Here too, prior art flame detection circuitry frequently has difficulty distinguishing a no-flame condition. For safety, such circuits must again err on the safe side, resulting in nuisance shutdowns which are not necessary.